Attention-Deficit/Hyperactivity Disorder (ADHD) has a male-to-female ratio ranging from 2:1 to 9:1, a ratio that suggests that boys and girls may follow different pathways or have differential vulnerability for the disorder. The proposed study would examine sex differences in neuropsychological and cognitive functioning, temperament traits, and sex hormone levels as they relate to ADHD symptom expression. Child participants between ages eight and thirteen will be recruited through public advertisements and outreach and will undergo a multistage screening process consisting of parent and teacher behavior rating scales and parent clinical interviews. Children will complete a battery of neuropsychological and cognitive tests to assess executive functions, delay aversion, intelligence, and achievement. Parents and children will complete temperament ratings to assess effortful and reactive control. Venipuncture blood draws will be obtained to assess estradiol and testosterone. Sample size will range from 150 to 585 with approximately thirty percent of the sample female. A key question addressed by regression analyses will be whether sex differences exist in the temperament and neuropsychological correlates of ADHD. Temperament and neuropsychological executive function will also be explored as potential mediators of the relationship between hormones levels and ADHD symptoms. These studies are novel and will provide new knowledge in a much theorized area. [unreadable] [unreadable] [unreadable]